Inspection devices capable of observing inside the human body in a minimally invasive manner, such as X-ray CT (computed tomography) and MRI (magnetic resonance imaging), have become widely available in recent years and been contributing significantly to medical diagnosis. Such apparatuses mainly provide a morphological image obtained by visualizing a tissue structure of the living subject as a tomographic image or volume data. In contrast, nuclear medical imaging, typified by PET (positron emission tomography) and SPECT (single photon emission CT), is a device for providing a functional image obtained by quantitatively visualizing physiological information such as glucose consumption, local blood flow, oxygen consumption, and the distribution of neurotransmitter receptors. With the recent increase of diseases such as cancers, dementia, and arteriosclerotic diseases, advances are being made in its research and clinical application. PET is also attracting attention as a powerful technique for promoting molecular imaging research for imaging the behavior of biomolecules.
PET/CT simultaneously capable of PET and X-ray CT imaging has recently been developed and become widespread in the clinical field. This has made possible diagnosis in light of both biological functions and body tissue. For example, in the diagnosis of cancer by PET, only the tumor portion is output with a high intensity. It may therefore be difficult to determine in which organ the tumor is. The superposition with X-ray CT, which provides excellent viewability of the form of the organs, is thus useful.
Instead of the combination with X-ray CT, PET/MRI to carry out diagnosis in combination with MRI has recently attracted attention. MRI not only can visualize the inside of living body with high spatial resolution, but also has the characteristics that the contrast of soft tissue is better than with X-ray CT, and that a functional image such as hemodynamic imaging and metabolic product concentration measurement by MR spectroscopy can be acquired. PET/MRI also has a lot of advantages, including that it is possible to avoid radiation exposure which is a problem with PET/CT. Its implementation is thus much expected (see Non-Patent Literature 1).
A PET device obtains information from annihilation radiations emitted from a radioactive drug reaching detectors. PET image reconstruction uses the detection data of annihilation radiations emitted in 180° directions. The annihilation radiations undergo attenuation while passing through various tissues of the body to reach the detectors. As a result, quantitative performance is greatly disturbed in deep portions of the subject. To obtain a quantitative drug distribution, the attenuation of the annihilation radiations needs to be corrected. A spatial distribution of radiation attenuation coefficient (p-map) needed for attenuation correction in the conventional PET image reconstruction is created on the basis of transmission measurement (referred to as transmission scan) separate from the data acquisition of the PET. The transmission scan is performed by rotating a radiation source 12 around the subject 10 and performing detection with a detector 14 as illustrated in FIG. 1 (see Patent Literatures 1 to 3). In the case of PET/CT, attenuation correction is usually performed by converting the X-ray CT image into a μ-map without the foregoing transmission scan.
MRI collects intensities obtained from protons (hydrogen nuclei) in tissues, and therefore cannot directly obtain the radiation attenuation rate of the respective tissues. Since the current design concept of PET/MRI does not include a transmission source similar to that of X-ray CT, a method for generating a μ-map as an alternative to the transmission scan is needed.
As a method for generating a μ-map by using an MR image, a segmentation method (see Non-Patent Literatures 2, 3, and 4) and a standard image reference method (see Non-Patent Literatures 5 and 6) have been proposed so far. In the segmentation method, as illustrated in FIG. 2, an MR image m is segmented into regions such as a high intensity region (soft tissue), a medium intensity region (water), and a low intensity region (air and bone) tissue by tissue, and μ values inherent to the tissues are substituted to generate a μ-map. In the standard image reference method, as illustrated in FIG. 3, a standard image (standard MR image ms or standard μ-map μs) is deformed to the MR image m of the patient by using affine transformation or the like.